1. Field of the Invention
This invention relates to integrated circuits and, in particular, to a homogeneous integrated power structure which embodies both control or signal devices and power devices integrated monolithically therein.
2. Description of the Prior Art
Heretofore, integrated circuits embodying both control signal devices (less than 500 volts and 1 ampere) and power devices (500 volts or greater and 1 ampere or greater) in the same structure have been difficult and costly to manufacture. High resistivity semiconductor material is employed as a common substrate for all devices to be made in the circuit. Any required buried layers of the circuit are formed by diffusion into the substrate. An epitaxial layer of semiconductor material of suitable resistivity and thickness is grown on the substrate to provide material for power devices. For a 600 volt power device, the epitaxial layer is approximately 65 microns in thickness and has a dopant concentration of approximately 3 .times. 10.sup.14 atoms per cubic centimeter. Suitable diffusion processes follow to provide electrical isolation and electrical contacts to the buried layers. One or more signal devices and the base and emitter for the power devices are then fabricated in the epitaxially grown material by diffusion processing techniques.
However, the high quality, high lifetime semiconductor material, of the order of about 100 microsecond, necessary for fabricating power devices is extremely difficult to grow epitaxially and prohibitively costly. The normal lifetime for epitaxial material on a degenerate substrate is of the order of from 1 to 5 microsecond. The effects of a low lifetime on power devices include high forward drops, V.sub.F, and lower voltage surge capability for semiconductor controlled rectifiers and rectifiers. In addition, transistors fabricated in the epitaxially grown semiconductor material have a low high current gain, h.sub.FE, high quasisaturation resistance, and less efficiency. Photo diodes fabricated in the same material have lower sensitivity and low efficiency ratings and contamination levels of less than 3 .times. 10.sup.14 atoms per cubic centimeter. Uncompensated material is probably unattainable on any reproducible scale and at any reasonable cost.
Additionally, it has been found by the prior art method that a vertical semiconductor controlled rectifier cannot be fabricated in the integrated circuit. The lateral semiconductor controlled rectifiers which are fabricated in the epitaxially grown material have high current densities which result from the flow of currents along restricted surface areas. Likewise, only lateral transistors are capable of being manufactured in the integrated circuit and a severe tradeoff must be made between the breakdown voltage and the current gain of the transistor. Power transistors must work at low current densities, and therefore, the physical dimensions of such transistors becomes very large to handle designed current requirements for the desired circuit.
In a co-pending patent application entitled "Homogeneous Integrated Power Structures", Ser. No. 526,226, filed the same day as this patent application and assigned to the same assignee, Krishna and Kurz teach the fabrication of power and signal or control semiconductor devices in the same body of semiconductor material. The devices fabricated in a homogeneous integrated power structure are electrically isolated from each other by one or more P-N junction regions. Each P-N junction region is formed by a double diffusion process step. The diffusion process time is of the order of 140 to 160 hours in length. A long furnace time at an elevated temperature of 1200.degree.C to 1250.degree.C is detrimental to the minority carrier lifetime and results in a decrease of the same. In addition, mismatches in the alignment of diffusion masks cause the P-N junctions regions to encroach upon the regions of material reserved for device manufacture. Further, the P-N junction regions which are formed are not uniform in resistivity throughout the region and have an hourglass configuration.
It is therefore an object of this invention to provide a new and improved integrated circuit embodying power and signal or control devices monolithically integrated into a common structure and electrically isolated from each other by one or more P-N junction regions having a substantially constant uniform width and resistivity throughout the region.
Another object of this invention is to provide high lifetime, high quality, and relatively inexpensive homogeneous semiconductor material in making new and improved integrated circuits embodying power and signal or control devices in a common monolithic substrate and electrically isolated from each other by one or more P-N junction regions having a substantially uniform width and resistivity throughout the region.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.